Millions of gallons of fuel oil and its equivalent are discarded every year through the disposal of plastic wastes and other waste material. Recycling of these wastes is of increasing importance as incineration and landfilling become more expensive and the acceptance of these methods is decreasing. It should be noted that rubber and plastic wastes are produced originally from crude oil and can be thermally cracked into fuels or petrochemicals. However, these wastes generally contain inorganic materials, fibers, glass, dust and poor thermal conducting materials, which are far more difficult to be treated effectively.
The quantity of organic-containing solid wastes increases rapidly at the rate of millions of tons per year. These organic-containing solid wastes are equivalent to approximately thousands of billions kcal, which is a huge amount of thermal potential energy/heat and about half are from petroleum products. These solid wastes such as: printed circuit board wastes, rubber wastes, plastic wastes, scrap tire, organic wastes from auto shredder residues, oil sludge/sediment etc., are usually mixed with inorganic materials i.e., iron-wires, metal, fiber, wood, glass. Generally organic matter pollutes the solid wastes and this increases the difficulty of resource recovering treatment.
In general, thermal treatment of wastes can recover energy and resources. These technologies include incineration, pyrolysis, oil liquefaction and gasification. Wastes incineration produces CO2 and H2O, but also produces some particulate, heavy metals, halides, SOx, and NOx. The accumulated pollutants have a negative impact on the environment. In addition the emission of PCCDS and PCDFS is also a serious problem. Under the condition of absence of oxygen, the macro organic compounds are cracked into smaller molecules and are recovered as the light hydrocarbons gases and light oil in the pyrolysis process. However, successful operation process for commercial purposes with direct pyrolysis are very few. This is due to engineering and operational problems such as (a) low heat transfer coefficient of the solid organic matter which affect the efficiency of the pyrolysis process; (b) high viscosity of products make the pyrolysis process more difficult; and (c) the pyrolysis products are not economically attractive, generally.
Many direct pyrolysis processes have been reported to have technical or economic difficulties. Indeed, pyrolysis is complicated by the fact that the polymeric material wastes are poor conductors and degradation of these macromolecules requires considerable amount of energy. The liquefaction process involves treating solid wastes with hot waste lubricating oil at temperatures between about 435-800° F. (about 225-425° C. and below general pyrolysis temperatures). Basically, organic macromolecules are soluble in heavy oils only if they are cracked effectively. Above about 435° F. (about 225° C.), the C—C bonds of the polymeric matrix can be disrupted and dissolved into the oil.
In a typical liquefaction process, solid wastes are liquefied in hot oil or recycled product heavy oils at relatively low temperatures. The liquefaction process comprises the main step of heat transfer by the hot oil to swell the structure of the highly polymeric organic material and lead to selective bond breaking. Therefore, the major products are oils, and can be separated easily from the mixtures of inorganic materials. The reaction temperature is usually less than about 750° F. (about 400° C.), which is much lower than that of any other known thermal treatment technologies. The need of the gas treatment equipment also becomes much less due to the lower quantity of the gas products. This proposed process can treat wastes mentioned above such as: printed circuit board wastes, scrap tire, plastic wastes, and other difficult-to-treat wastes as well as used motor oil. Hence the goal of treating several kinds of wastes simultaneously can be achieved.
The liquefaction process usually takes place in a liquefaction reactor, where the solid wastes are dissolved and suspended into the hot oil or heavy recycled product oils. While the agitation requirements of this process have been demonstrated to be undemanding, the high viscosity of the mixture in the liquefaction reactor, the presence of non-soluble polymers and inorganic debris, and the tendency of any unmelted feed waste to float on the viscous fluid surface must all be accounted for.
In small-scale liquefaction processes, such as on the pilot plant scale, liquefaction reactors are small enough that screens can be used to filter out the solid material from the liquefied waste. In large-scale commercial plants, screens are generally not used. Thus, there are concerns about the behavior of solid materials in those commercial plants. Additionally, unmelted waste small-scale units generally don't have the floating problems that commercial scale plants have. Thus commercial plant liquefaction reactors must have an agitation device that is better suited to drawing unmelted waste below the liquid surface, while minimizing the amount of gas drawn below the liquid surface and the depth that trapped gas is forced to.